[PATCH] ethernet:enic: Fix a use after free bug in enic_hard_start_xmit

[Lv Yunlong]

In enic_hard_start_xmit, it calls enic_queue_wq_skb(). Inside
enic_queue_wq_skb, if some error happens, the skb will be freed
by dev_kfree_skb(skb). But the freed skb is still used in
skb_tx_timestamp(skb).

My patch makes enic_queue_wq_skb() return error and goto spin_unlock()
incase of error. The solution is provided by Govind.
See https://lkml.org/lkml/2021/4/30/961.

Fixes: fb7516d42478e ("enic: add sw timestamp support")
Signed-off-by: Lv Yunlong <lyl2019@xxxxxxxxxxxxxxxx>
---
 drivers/net/ethernet/cisco/enic/enic_main.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/drivers/net/ethernet/cisco/enic/enic_main.c b/drivers/net/ethernet/cisco/enic/enic_main.c
index f04ec53544ae..40abc3fdeba6 100644
--- a/drivers/net/ethernet/cisco/enic/enic_main.c
+++ b/drivers/net/ethernet/cisco/enic/enic_main.c
@@ -768,7 +768,7 @@ static inline int enic_queue_wq_skb_encap(struct enic *enic, struct vnic_wq *wq,
 	return err;
 }
 
-static inline void enic_queue_wq_skb(struct enic *enic,
+static inline int enic_queue_wq_skb(struct enic *enic,
 	struct vnic_wq *wq, struct sk_buff *skb)
 {
 	unsigned int mss = skb_shinfo(skb)->gso_size;
@@ -813,7 +813,9 @@ static inline void enic_queue_wq_skb(struct enic *enic,
 		}
 		wq->to_use = buf->next;
 		dev_kfree_skb(skb);
+		return err;
 	}
+	return err;
 }
 
 /* netif_tx_lock held, process context with BHs disabled, or BH */
@@ -857,7 +859,8 @@ static netdev_tx_t enic_hard_start_xmit(struct sk_buff *skb,
 		return NETDEV_TX_BUSY;
 	}
 
-	enic_queue_wq_skb(enic, wq, skb);
+	if (enic_queue_wq_skb(enic, wq, skb) < 0)
+		goto error;
 
 	if (vnic_wq_desc_avail(wq) < MAX_SKB_FRAGS + ENIC_DESC_MAX_SPLITS)
 		netif_tx_stop_queue(txq);
@@ -865,6 +868,7 @@ static netdev_tx_t enic_hard_start_xmit(struct sk_buff *skb,
 	if (!netdev_xmit_more() || netif_xmit_stopped(txq))
 		vnic_wq_doorbell(wq);
 
+error:
 	spin_unlock(&enic->wq_lock[txq_map]);
 
 	return NETDEV_TX_OK;
-- 
2.25.1